The international Comprehensive Nuclear-Test-Ban Treaty (CTBT) includes a verification regime to detect any nuclear explosion in the world. Part of the verification regime includes the monitoring and detection of radionuclides in the atmosphere. Radionuclides of interest include radioactive isotopes of the noble gas xenon (e.g., 135Xe, 133Xe, 133mXe, and 131mXe). Noble gas collection and radionuclide measurement systems include radiation detectors that require calibration to verify the systems are working properly and providing accurate quantitative results. Calibration includes exposing the systems to gas samples that include a known content of a radionuclide of interest. Standard gas samples used for calibration include radioxenon (i.e., radioactive isotopes of xenon) mixed with stable xenon (i.e., non-radioactive isotopes of xenon) or radioxenon mixed with air (for example, approximately 87 ppb radioxenon in air). Measuring radioxenon in stable xenon involves a small volume of gas with a relatively easily detectable radioactive signal. However, the detection of radionuclides in the atmosphere for the CTBT or for other purposes involves the detection of radionuclides in ambient air, hereinafter referred to as “air” for convenience.
Measuring radioxenon in air involves a large volume of gas with a relatively small radioactive signal due to the low concentration of radioxenon. Accurate calibration and direct measurement of radioxenon in air is difficult based on several factors. First, radioxenon isotopes have a short half-life, which makes detecting radioxenon over a long period of time difficult, if not impossible. Second, large-volume, low-pressure samples include outer portions of the samples that are far from the radiation detector, resulting in low measurement efficiency. Third, although small-volume, high-pressure samples provide a more concentrated and higher signal for measuring, such samples require thick-walled containers to attain and maintain a high pressure. The thick walls attenuate (i.e., block) measurable gamma signals emitted from the radioxenon. While extraction of radioxenon from air for measurement is possible, it remains difficult to accurately quantify the amount of air processed and separated from the extracted radioxenon. Thus, it is difficult to accurately estimate the radioxenon concentration in the original sample.
The measurable intensity of gamma signals from a sample of radioxenon (or other radionuclides) in air is reduced by attenuation from the air itself and from any barrier between the sample and the radiation detector. Attenuation is a function of a distance that the gamma signals travel from the sample to the radiation detector in addition to any barrier that the gamma signal must pass through. Some existing radionuclide measurement systems include gas containers (e.g., Marinelli-type containers) having sidewalls fabricated from thin plastic to reduce attenuation. Marinelli-type containers generally include an outer shell, an inner shell for receipt of a radiation detector, and a space between the outer shell and inner shell for containing a sample fluid. However, such existing plastic, gas containers for detection of radionuclides have a relatively low maximum operating pressure (e.g., 10 psi) and are generally not capable of use for detection of low activity, low energy, and high pressure gases, such as radioxenon in compressed air.